Hydroxamic acids (N-hydroxyarylamides) have been implicated in various toxicities including the carcinogenicity produced by 2-acetylaminofluorene and other poly-cyclic N-acetylarylamides and the cell necrosis produced acetaminophen and phenacetin. These compounds, however, are very difficult to assay by conventional methods. A specific high pressure liquid chromatographic assay for hydroxamic acids has thus been developed. In this assay ferric chloride is included in the elution solvent and the hydroxamic acids are isolated and detected as ferric chelates at 546 nm. The assay is very sensitive with a number of hydroxamic acids including those of 2-acetylaminofluorene, phenacetin, acetaminophen and acetanilide. There was only minimal interference from isomeric phenols.